The present invention relates to a focussed ion beam (FIB) apparatus using a liquid metal ion source (LMIS).
In the conventional FIB apparatus, the ion beam current is stabilized as follows. As shown in FIG. 2, an ion source 1 is supplied with power from a heater source circuit 8 and thus emits an ion beam 4. A beam current from a monitor electrode 3 is fed back, via a differential amplifier 7 connected to receive an adjustable comparison, or reference, input, to a beam extraction voltage circuit 5 to keep the amount of current flowing into the monitor electrode 3 constant. This is an analog feedback. The analog feedback, which is fast, is capable of sufficiently stable control if the emission conditions of the LMIS do not change.
With LMIS, however, when ions emitted from the end of a needle source 1 strike the extraction electrode 2, sputtered metal atoms released from electrode 2 are adsorbed on the needle end. In addition, gas molecules may also attach to the needle end, contaminating it. As a result, the liquid metal ion flow may change, causing a change in the emission conditions.
At this time, with a simple analog feedback, the beam extraction voltage being monitored is stable, as shown in region A of FIG. 3, before an emission condition changes. When the emission condition changes, the beam extraction voltage increases as in region B or oscillates as in region C. During operation in regions B and C, precision machining or observation is not possible.